1. Technical Field
The present invention relates to a printing device such as a facsimile device, a copying machine, or a printer for office automation equipment and, more specifically, to a printing device, a printing program, a printing method, an image processing-device, an image processing program, an image processing method, and a recording medium in which the program is stored suitable for performing a printing process of a so-called inkjet system, in which predetermined characters or images are drawn on a printer sheet (recording material) by discharging fine particles of liquid ink in a one or more colors.
2. Related Art
A printer, and more specifically, a printer in which such an inkjet system is employed (hereinafter, referred to as “inkjet printer”) will be described below.
Inkjet printers are widely used not only in offices, but also among general users in tandem with the popularity of personal computers and digital cameras since they are generally cost effective and can easily provide a high-quality color printing.
An inkjet printer is adapted to create a desired printed material by moving a movable member including an ink cartridge and a print head integral therewith, which is called a “carriage” or the like, over a printing medium (paper) reciprocally in a direction vertical to a paper-feeding direction while discharging (injecting) particles of liquid ink from a nozzle of the print head in dots, thereby drawing predetermined characters or images on the printer sheet. When four of such ink cartridges for four color printing including black (yellow, magenta, cyan), and the print heads for the respective colors are provided on the carriage, not only monochrome printing, but also full color printing can be easily achieved by combining these colors (in addition, combinations of six, seven, and eight colors with light cyan or light magenta added thereto have also come into practical use).
In the inkjet printer of the type in which printing is executed by moving the print head on the carriage reciprocally in the direction vertical to the paper-feeding direction (a widthwise direction of the printer sheet), it is necessary to cause the print head to reciprocate from several tens of times to more than one hundred times in order to achieve a good-looking printing on one page. Therefore, it has a drawback such that a significantly long printing time is required in comparison with a printing device of other systems, such as a laser printer in which an electrophotographic technology such as a copying machine is employed.
In contrast, in the inkjet printer of a type in which an elongated print head having the same (or larger) length as the width of the printer sheet is arranged so that the carriage is not used, and hence it is not necessary to move the print head in a widthwise direction of the printer sheet. Therefore, a so-called single-scan (single-pass) printing is achieved, and hence high-speed printing as with the laser printer is enabled. In addition, since the carriage to mount the print head and a drive system for moving the same are not necessary, reduction of the size and weight of an enclosure of the printer is possible. Furthermore, noise reduction is significantly improved. The inkjet printer of the former type is generally called a “multi-pass type printer” and the one of the latter type is generally called a “line-head type printer”.
The print head which is essential in the inkjet printer includes minute nozzles on the order of 10-70 μm in diameter at predetermined intervals arranged in series or in a plurality of rows in the printing direction. Therefore, for example, there may be a case in which directions of ink discharge from some of the nozzles are angled or the positions of the nozzles are arranged at positions deviated from ideal positions due to manufacturing error and, consequently, landing positions of dots formed on the printing medium by these nozzles are deviated from ideal positions, which is called a “discharge deviation phenomenon”. Due to such non-uniform characteristics of the nozzles, those which vary widely from others may discharge much more or much less ink in comparison with an ideal amount.
Consequently, there is a case in which defective printing results, which is called a “banding phenomenon”, at a part printed by the defective nozzles and hence the printing quality is significantly lowered. In other words, when the discharge deviation phenomenon occurs, distances between dots discharged from adjacent nozzles become uneven. Parts in which the distances between the adjacent dots are longer than the normal distance, “white bands” (when the printer sheet is white) are generated, and parts in which the distances of the adjacent dots are shorter than the normal distance, “dark bands” are generated. In a case in which the value of the amount of ink is deviated from the ideal value, the dark bands are generated at positions of the nozzles which discharge the larger amount of ink and the white bands are generated at positions of the nozzles which discharge the less amount of ink.
In particular, such a banding phenomenon tends to occur in the “line-head type printer” in which the print head or the printing medium is fixed (single-pass printing) in comparison with the above-described “multi-pass type printer” (The multi-pass type printer has a technique that reduces the banding phenomenon to an invisible level using a technique of reciprocating the print head many times).
Therefore, in order to prevent a sort of defective printing due to the “banding phenomenon”, study and development in a way pertaining to hardware such as improvement in technology of manufacturing the print head or improvement of design have been carried out. However, it is difficult to provide a print head in which the occurrence of the “banding phenomenon” is completely eliminated because of the manufacturing cost and technological limitations.
Therefore, in the status quo, in addition to the improvement in a way pertaining to hardware as described above, a technology to reduce the “banding phenomenon” in a way pertaining to software, such as printing control as shown below is employed in parallel.
In order to cope with fluctuations of the nozzles or non-discharging of ink, for example, in JP-A-2002-19101 and JP-A-2003-136702, a shading correction technique is used for portions with less density to cope with the fluctuations of the heads, and other colors are used for portions of high density to reduce the banding or fluctuations to an invisible level.
In JP-A-2003-63043, in a case of solid color images, a method of increasing amounts of discharge from adjacent nozzles corresponding to proximal pixels of a non-discharge nozzle to form a solid color image with all the nozzles in cooperation is employed.
In JP-A-5-30361, an attempt is made to avoid the banding phenomenon by feeding back the amount of variations of the respective nozzles to an error diffuser to accommodate variations in the amount of ink discharge among the nozzles.
However, in the method of alleviating the banding phenomenon or fluctuations by using other colors as in JP-A-2002-19101 or JP-A-2003-136702, a color hue of parts applied with such processing may vary, and hence it is not suitable for printing images such as a color photo image in which high definition and high quality are required.
When the method of allocating information of non-discharging nozzle to left and right nozzles for the portion of high density to avoid the “white banding phenomenon” is applied to the discharge deviation phenomenon described above, the white bands can be reduced. However, the banding disadvantageously remains in the part with high density.
On the other hand, the method disclosed in JP-A-2003-63043 is effective when printed material is a solid color image. However, this method cannot be applied when it is of intermediate gradations. In a case of a thin line, a method of substituting for the missing color by other colors can be employed without problem as long as it is a very small amount. However, in a case of an image in which other colors appear continuously, there remains a problem that the color hue of the image is partly varied as in the former case.
The method disclosed in JP-A-5-30361 can avoid the banding phenomenon caused by the amount of ink discharge from the nozzles. However, as regards the problem of the banding phenomenon caused by positional displacement of dot formation, adequate feedback is difficult.